1. Field of the Invention
The present invention relates to a technique for searching corresponding points from two pieces of image information obtained from a stereo camera.
2. Description of the Background Art
A method is known in a stereo camera that each coordinate point on two images captured from different viewpoints are associated with each other and a three-dimensional position of each coordinate point is calculated from correspondence relation obtained on the basis of the principle of triangulation.
For example, there is a method of obtaining degree of similarity (correlation value; COR) between an image in a window set on an image (base image) and an image in each of a plurality of windows set on another image (reference image) having the same size as the window and searching corresponding points in the two images by specifying the window position on the reference image having the highest degree of similarity. By using the corresponding point searching method, the three-dimensional position of each of points on an image captured by a stereo camera can be obtained.
With respect to the methods as described above, in recent years, attention is paid to corresponding point searching using Phase Only Correlation (POC) method capable of performing robust and high precision associating. In the method, amplitude components in images are eliminated and correlation calculation is performed with only phase components in the images. The method is therefore not easily influenced by brightness fluctuations and noise.
Japanese Patent Application Laid-Open No. 10-134196 discloses a method capable of measuring a distance to a target subject in an image by using POC. A method using POC for associating coordinate points between images is disclosed in Kenji Takita, et al., “High-Accuracy Subpixel Image Registration Based on Phase-Only Correlation”, IEICE Trans. Fundamentals, Vol. E86-A, No. 8, August 2003.
In the technique disclosed in Japanese Patent Application Laid-Open No. 10-134196, it is assumed that only one window is set in a distance measurement target portion in an image, so that processing time is not an issue. However, as described in Kenji Takita, et al., “High-Accuracy Subpixel Image Registration Based on Phase-Only Correlation”, IEICE Trans. Fundamentals, Vol. E86-A, No. 8, August 2003, in the case of associating coordinate points between images, a problem that processing time is very long arises.
To address such a problem, two cameras to form the stereo camera are mechanically disposed almost parallel with each other (or images arranged almost in parallel with each other are obtained by an image processing). Since a correspondence position on a reference image corresponding to a coordinate point on a base image exists in a direction almost parallel with a base line length, the processing can be performed at high speed by using, for example, an SAD (Sum of Absolute Differences) method or the like.
However, even if the two cameras are disposed in a position almost parallel with each other initially, in the case where the two cameras are deviated from the parallel position due to a change with time or the like, a problem occurs such that precision largely deteriorates.